1. Field of the Invention
The present invention relates in general to a method and a circuit for driving a liquid crystal display (LCD). In particular, the present invention relates to a method for driving an LCD having display cells in the same row sharing a data electrode to eliminate cross talk and improve image appearance.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a prior art liquid crystal display panel (hereinafter, referred to as an “LCD panel”) and the peripheral driving circuits thereof. As shown in the figure, an LCD panel 1 is formed by interlacing data electrodes (represented by D1, D2, D3, . . . , Dm) and gate electrodes (represented by G1, G2, G3, . . . , Gm), each of the interlacing data electrodes and gate electrodes controlling a display cell. As an example, interlacing data electrode D1 and gate electrode G1 control the display cell 200. The equivalent circuit of each display cell comprises thin film transistors (TFTS) (Q11–Q1m, Q21–Q2m , . . . , Qn1–Qnm) and storage capacitors (C11–C1m, C21–C2m , . . . , Cn1–Cnm). The gates and drains of these TFTs are respectively connected to gate electrodes (G1–Gn) and data electrodes (D1–Dm). Such a connection can turn on/off all TFTs on the same line (i.e. positioned on the same scan line) using a scan signal of gate electrodes (G1–Gn), thereby controlling the video signals of the data electrodes to be written into the corresponding display cell. It is noted that a display cell only controls the brightness of a single pixel on the LCD panel.
Accordingly, each display cell responds to a single pixel on a monochromatic LCD while each display cell responds to a single subpixel on a color LCD. The subpixel can be red (represented by “R”), blue (represented by “B”), or green (represented by “G”). In other words, a single pixel is formed by an RGB (three display cells) combination.
In addition, FIG. 1 also shows a part of the driving circuit of the LCD panel 1. The gate driver 10 outputs one or more scan signals (also referred to as scan pulses) of each of the gate electrodes G1, G2, . . . , Gn according to a predetermined sequence. When a scan signal is carried on one gate electrode, the TFTs within all display cells on the same row or scan line are turned on while the TFTs within all display cells on other rows or scan lines may be turned off. When a scan line is selected, data driver 20 outputs a video signal (gray value) to the m display cells of the respective rows through data electrodes D1, D2, . . . , Dm according to the image data to be displayed. After gate driver 10 scans n rows continuously, the display of a single frame is completed. Thus, repeated scans of each scan line can achieve the purpose of continuously displaying an image. As shown in FIG. 1, signal CPV indicates the clock of the gate driver 10, signal CTR indicates the scan control signal received by the gate driver 10, signal LD indicates a data latch signal of the data driver 20, and signal DATA indicates the image signal received by the data driver 20.
Typically, a video signal, which is transferred by the data electrodes D1, D2, . . . , Dm, is divided into a positive video signal and a negative video signal based on the relationship with the common electrode voltage VCOM. The positive video signal indicates a signal having a voltage level higher than the voltage VCOM, and based on the gray value represented, the actual produced potential of the signal ranges between voltages Vp1 and Vp2. FIG. 2 shows the relationship between the common electrode voltage VCOM and the voltages VP1, Vp2, Vn1 and Vn2. In general, a gray value is lower if it is closer to the common electrode voltage VCOM. On the other hand, the negative video signal indicates that the signal has a voltage level lower than the voltage VCOM, and based on the gray value represented, the actual produced potential of the signal ranges between voltages Vn1 and Vn2. Also, the gray value is lower if it is closer to the common electrode voltage VCOM. When a gray value is represented, whether in a positive or negative video signal, the display effect is substantially the same.
In order to prevent the liquid crystal molecule from continuously receiving a single-polar bias voltage, thus reducing the life span of liquid crystal molecules, a display cell alternately receives positive and negative polar video signals corresponding to odd and even frames.
The disposition of the different polar video signal in each display cell can be divided into four driving types: frame inversion, line inversion, column inversion, and dot inversion. In the frame inversion driving mode, the polarity of the video signal is the same on the same frame but opposite to its adjacent frames. In the line or column inversion driving modes, the same line or column on the same frame has the same polarity of the video signal but the opposite polarity to its adjacent lines or columns. In the dot inversion driving mode, the polarity of the video signal on the same frame is presented in an interlaced form, which is the type described in the present invention.
FIG. 3 shows the polarity of the video signals received by each display cell on the LCD panel 3 in dot inversion driving mode. In FIG. 3, the LCD panel 3 comprises a plurality of display cells. The display cells responding to the same gate electrode are connected to different data electrodes, respectively. In dot inversion driving mode, the polarity of each display cell opposite to its adjacent display cells connected to the same gate electrode or data electrode in a frame.
As mentioned above, when a gray value is represented, whether in a positive or negative video signal, the display effect is substantially the same. In addition, the number of the positive video signals and negative video signals received by a data electrode is the same. Thus, the common electrode voltage VCOM is not obviously shifted.
FIG. 4 shows the polarity of the video signals received by each display cell on the LCD panel 3 in dot inversion driving mode when a plurality of display cells in one row sharing a single data electrode. The number of the data electrodes is decreased when there are some display cells in one row sharing a single data electrode. Thus, the area of the data electrode is also decreased. In dot inversion driving mode, when there are n display cells in the same row sharing one data electrode, the data electrode D0 drives the first display cell, and then the data electrode D1 drives the (n+1)th display cell in opposite polarity. After the data electrode Dm driving the (mn+1)th display cell, the data electrode D0 drives the second display cell and then the data electrode D1 drives the (n+2)th display cell in opposite polarity. Here, the display cells in the same row sharing one data electrode comprise a display group. However, the common electrode voltage VCOM is shifted by electric coupling.
Although the driving method shown in FIG. 4 sequentially drives display cells in opposite polarity, the adjusted display cells are not driven in sequence because the driving method is changed. Thus, in a display group, the number of the display cells with positive polarity may be different than the display cells with negative polarity. Therefore, the common electrode voltage VCOM is from shifting by coupling and the gray levels of the display cells are incorrect. As shown in FIG. 4, all the display cells in a display group have the same polarity. Thus, the common electrode voltage VCOM is shifted by electric coupling.